Numerous key chain designs are in use today, from simple key rings to deluxe key cases finished to match accessory items, such as wallets and purses. Some are designed to include two ring portions which can be separated temporarily. With such a two ring key chain, keys can be divided logically with one group of keys, such as keys for the home, on one ring and another group of keys, such as car or office, on the second ring.
The ability to separate home or office keys from car keys provides an extra measure of safety which often comes in handy when leaving a car for service, or with valet parking, etc. Doing so greatly reduces the chances of home or office keys falling into the wrong hands. Most people probably consider separating car keys from the rest of their keys when leaving their car with a stranger; making this easier to do increases the likelihood of actually following through with the idea.
Key chains are usually carried in a pocket or purse, where space is often at a premium; many people prefer to "travel light", and take the attitude that the fewer items carried, the better. One means of achieving this goal is to combine two or more items, which are usually carried separately, in a single container.